Substituting Fish Meal with a Bacteria Protein (Clostridium autoethanogenum Protein) Derived from Industrial-Scale Gas Fermentation: Effects on Growth and Gut Health of Juvenile Large Yellow Croakers (Larimichthys crocea)

Abstract

This study was conducted to investigate the effects of Clostridiumautoethanogenum protein (CAP) replacement for fish meal (FM) on growth performance, digestive enzyme activity, humoral immunity and liver and intestinal health in large yellow croakers (Larimichthys crocea). Four experimental diets were formulated by replacing FM with CAP at different levels—0% (CAP0), 15% (CAP15), 30% (CAP30) and 45% (CAP45). Triplicate groups of juveniles (initial weight = 11.86 ± 0.13 g) were fed the test diets to apparent satiation two times daily for eight weeks. There was no significant difference in final body weight (FBW), weight gain rate (WG) and feed efficiency (FE) between CAP0 and CAP15. However, compared to the CAP0, CAP30 and CAP45 significantly reduced FBW, WG and LDR (p < 0.05), while CAP45 significantly reduced FE and PDR (p < 0.05). The whole-body moisture was significantly increased by CAP replacement of FM while crude lipid content was decreased (p < 0.05). No significant difference in crude protein, ash and liver crude lipid was observed among all groups (p > 0.05). Compared to CAP0, CAP30 and CAP45 significantly reduced serum C4 concentration (p < 0.05), and CAP45 significantly reduced serum AKP activity (p < 0.05) but significantly increased LZM activity (p < 0.05). Serum C3 concentration was significantly increased by CAP15 (p < 0.05). In terms of intestinal histology, CAP addition significantly increased the thickness of intestinal villus (p < 0.05), and CAP15 and CAP45 significantly increased the thickness of intestinal muscular (p < 0.05). The addition of CAP significantly reduced serum DAO and D-lactate concentrations (p < 0.05), indicating the intestinal physical barrier was improved. The results of 16S rRNA gene sequencing showed that the intestinal microorganisms of large yellow croakers are dominated by organisms from Proteobacteria, Bacteroidetes and Firmicutes. The addition of CAP reduced the relative abundance of Ralstonia and Christensenellaceae and increased the relative abundance of Paenibacillus. Overall, the optimum level of CAP replacement FM in large yellow croakers feed is 15%, which helps to improve humoral immunity and intestinal health with no adverse effects on growth. However, the 30% and 45% substitution levels adversely affect the growth and humoral immunity of large yellow croakers.